9. COLLIDING GALAXIES:

Galaxies in clusters are close enough and big enough that they often pass nearby each other, as illustrated above. In that case, the gravity due to a passing galaxy can perturb the orbits of the stars about the center of the other galaxy, causing great tidal distortions of the shape of each galaxy. In fact, we now know that these tides can easily cause new spiral arms to form in a disk galaxy. Hundreds of millions of years thereafter, the spiral arms may wind up more and more tightly, as the galaxy changes its morphology from type Sc to Sb to Sa. For example, the spiral arms of the Milky Way may be a result of gravitational tides due to its satellite, the Large Magellanic Cloud. Another example is shown below.

In fact, sometimes galaxies will "collide," passing right through each other. I put "collide" in quotes because it is unlikely that any stars will actually hit each other during such collisions, since interstellar distances are so great. But the collisions will greatly disturb the orbits of the stars, leaving highly deformed galaxies.

The hydrogen gas in spiral galaxies can often be seen at the outer reaches of the galaxy, beyond the visible stars. By observing this gas with radio telescopes, one can sometimes trace the history of the collisions of the galaxies. You can find a very good example in Min Yun's M81 HI Home Page.

More than 300 years ago, Isaac Newton wrote down the equations that describe the motions of stars and planets due to their mutual gravity, and his solutions describe the orbits of the planets and comets very well. We can use these same equations to describe the collisions of galaxies. But it is much more difficult to solve Newton's equations when we must describe the motions of billions of stars all tugging on each other instead of just one planet or comet tugged by the gravity of the Sun. But today, astrophysicists have learned to simulate collisions of galaxies by solving such equations on the world's largest computers.

Here's a movie (source) by Joshua E. Barnes of the University of Hawaii illustrating a simulation of the collision of two counter-rotating spiral galaxies. This particular simulation produces a pair of interacting galaxies that appear very similar to the antennae. This collision was a few hundred millions of years. If this movie continued, you would see the two galaxies merge together into a single galaxy that would resemble an elliptical galaxy. Here's a sequence of images of galaxies representing various stages of such a collision. We can estimate the time since the collision began for each of these images from the colors of their globular star clusters. In the early stages of the collision the star clusters are blue because they have many newly-formed blue giants, while in the late stages the clusters are red because all the blue stars have evolved and become red giants (see Lesson 5).

Although the stars in galaxies miss each other when galaxies collide, the interstellar gas in the two galaxies, which fills the space between the stars, must crash. When it does, the gas is compressed and this compression can trigger a huge outburst of star formation called a starburst. We see an example of collision-induced star formation in the Hubble image of the Antennae galaxies above. (source: Hubble Reveals Stellar Fireworks Accompanying Galaxy Collisions).  

Another interesting example of star formation caused by collisions of galaxies is the Cartwheel Galaxy, shown above. In this case, a smaller galaxy (probably the blue one on the right) passed almost straight through the center of the larger one on the left, sending an expanding circular compression wave through its interstellar gas. New stars are being formed in this big "splash".

Because star-forming regions contain many luminous blue and red supergiant stars, starburst galaxies are exceptionally luminous, especially at infrared wavelengths. They are more common in the early universe, when galaxies were colliding more often than they are today.

One of the few galaxies that is moving toward the Milky Way is M31, the other giant spiral galaxy in the Local Group. Perhaps in a few billion years M31 will crash into the Milky Way. Here's a computer simulation of the Impact of the Milky Way and M31 by John Dubinski.

Want to have some fun: simulate your own galaxy collisions with Galaxy Crash, by Greg Bothun and Chris Mihos.

When a small galaxy collides with a large galaxy, the large galaxy merges with it in a process called cannibalism. When it absorbs the smaller galaxy, the larger galaxy will swell up somewhat because it is "heated" by the energy of the high-speed stars from the small galaxy. Such cannibalism occurs commonly in clusters of galaxies and is responsible for the existence of very massive galaxies often found at the centers of such clusters. Here's a computer simulation of this process by John Dubinski. A good example of a big fat cannibal that has obviously eaten many smaller galaxies is the giant elliptical M87 in the Virgo Cluster of galaxies, which we have already discussed in the context of its X-ray emission. For more examples, see Hubble Sees More Evidence of Galactic Cannibalism and A Collision in the Heart of a Galaxy.

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Last modified March 30, 2002
Copyright by Richard McCray